1. Field of the Invention
The present invention relates to a green phosphor for a Plasma Display Panel (PDP), and more particularly to a green phosphor for a PDP having an improved life-span and discharge stability.
2. Description of the Related Art
A Plasma Display Panel (PDP) is a flat display device using a plasma phenomenon, which is also called a gas-discharge phenomenon in which a discharge is generated in the PDP when a potential greater than a certain level is applied to two electrodes separated from each other in a gas atmosphere in a non-vacuum state. Such a gas-discharge phenomenon is applied to display an image in the PDP.
The currently generally used PDP is an Alternating Current (AC) driven PDP. The AC PDP has a structure in which a front substrate is disposed facing a rear substrate, with a discharge space between the two substrates. On the front substrate, a pair of retaining electrodes (scan electrode X, common electrode Y) are arranged in a certain pattern, each composed of a transparent electrode and a metal film. A dielectric layer is also coated thereon for the AC driving. The surface of the dielectric layer is coated with an MgO passivation layer. On the rear substrate, an address electrode A, a dielectric layer, a barrier rib, and a phosphor layer (R, G, B) are arranged.
The front substrate is disposed facing the rear substrate and is sealed. The internal space thereof is evacuated to reach a vacuum state, and the discharge gas is injected therein. The discharge gas may include any one or a mixture of inert gasses such as He, Ne, or Xe. Such a PDP includes three electrodes in its discharge space and a phosphor layer (R, G, B) which is as an array of red, green, and blue phosphor patterns. When a predetermined voltage is applied across the two electrodes to induce a plasma discharge, the fluorescent layer is excited by UV rays generated by the plasma discharge and emits light.
Typically, the phosphor used for the PDP is a phosphor that is excited by ultraviolet rays. As the green has the highest fraction on white brightness among R, G, and B, the green brightness is the most important for improving the PDP brightness. Currently, Zn2SiO4:Mn, BaAl12O19:Mn, (Ba,Sr,Mg)O.aAl2O3:Mn (a is an integer of 1 to 23) are used for the green phosphor, and Zn2SiO4:Mn is the most popular due to its better brightness characteristics. However, it also has a defect in that the discharge characteristics are degenerated. The reason why the discharge characteristics of Zn2SiO4:Mn are degenerated will now be described in detail.
Since the MgO layer of the front substrate and the phosphor layer R, G, B of the rear substrate are directly exposed to the discharge space, the secondary electron emission coefficient of the MgO layer and the surface charge of the phosphor layer are directly affected by the amount of wall charge piled up on the phosphor layer and the MgO layer. During positive surface electrification, discharge failure is rarely generated, while during the negative surface electrification, discharge inferiority is frequently generated. This tendency is deeply dependant on the driving system. In order to increase the discharge stability and to decrease the discharge inferiority, it is preferable to select the R, G, B phosphor so that the surface electrification characteristic is positive regardless of the R, G, B color. Nevertheless, Zn2SiO4:Mn, the most popular green phosphor, has a negative surface electrification characteristic. Accordingly, when the PDP is driven in a driving waveform sensitive to the surface electrification characteristics of the phosphor layer, that is, the variation of the rear substrate, the discharge voltage of the green cell is higher than those of the red cell and the blue cell.
The mechanism to increase the discharge voltage may be described as follows: upon the reset discharge, the characteristic of driving an AC PDP during the real discharge, that is, before the discharge voltage is applied to the address electrode terminal, the wall charge is piled up. Before the discharge voltage is applied to the address electrode terminal, the wall charges having counter polarities are respectively piled up on the front substrate and the rear substrate. Thereby, a voltage differentiation is generated between the front and rear substrates.
Upon the voltage differentiation reaching a certain level, a voltage having the same polarity as the wall charge piled up on both the address electrode terminal and the scan electrode terminal is applied to discharge. Thus, the address discharge voltage is lowered by effectively piling the wall charge at an appropriate level. Before the discharge voltage is applied to the address electrode terminal, the cations pile up on the surface of the phosphor layer of the rear substrate as a wall charge. As the Zn2SiO4:Mn having negative surface electrification characteristics is counterbalanced by the wall charge of cations, the green cell generates a smaller discharge voltage that those of the red cell and blue cell. Accordingly, the green cell of Zn2SiO4:Mn may require a higher address voltage compared to the cases of the red cell and the blue cell, and sometimes, a discharge failure is generated.
In order to solve the problems relating to Zn2SiO4:Mn, Korean Laid-Open Patent Publication No. 2001-62387 relates to a green phosphor in which YBO3:Tb is added to Zn2SiO4:Mn. However, the obtained green phosphor has deteriorated color purity. Furthermore, Korean Laid-Open Patent Publication No. 2000-60401 relates to a green phosphor in which a positive charged material of zinc oxide and magnesium oxide is added to Zn2SiO4:Mn. However, the green phosphor obtained from this method also causes problems in that the color purity and the lifespan are deteriorated. Still furthermore, Japanese Laid-Open Patent Publication No. 2003-7215 discloses that a mixture of manganese-activated aluminate green phosphor and terbium-activated phosphate or terbium-activated borate green phosphor can improve the driving voltage and the brightness failure.